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measuring CV magnitudes

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spp
spp's picture
measuring CV magnitudes

Is it possible to use VPhot to measure unfiltered CV magnitudes (e.g.  V380 Oph)?  If so, what should be placed in the "filter mapping rule"?

Phil

Lew Cook
Lew Cook's picture
CV Magnitudes

Phil, I do it quite a lot for CV's. Since they have B-V and V-I ~0 (usually), I use VPHOT and in the box for what sequence to use, I put "V",  click REFRESH, and get a pseudo "V" (which is what CV is). I'd not try it with a red star!

Anyone else have 2c?

Lew

Lew Cook
Lew Cook's picture
2 cents more

The reason for doing things in CV (i.e. unfiltered) rather than with a photometric filter is:

1. You can get fainter stars in the same length of exposure (no matter how big your telescope is). Yes, the bigger your telescope, the fainter you can get.

2. You can get quicker cadence if you suspect the star is varying rapidly.

3 You will get a firmer grasp of the light curve and its intricacies with more points in the curve.

4. The error bars will be smaller because the S/N is greater with unfiltered data.

5. Filters have one purpose: to throw away the light that doesn't meet the criteria  for which the filters were designed.

6. You must accept the fact that your data WILL have a systematic error.

From all this, if you need big signals from tiny specks on the CCD, unfiltered is the way to go. Or buy a larger telescope. Some photometric studies, probably most, require photometric systems. A few can get away without filters. It IS always reassuring to have some B, V and I data in the mix of data!

I'm sure that Arne has some thoughts on the heresy I have just expressed. Arne?

MZK
MZK's picture
CV Magnitudes in VPhot

Phil:

It is quite easy to use Clear filters for your imaging and report results as CV, which is what AID would like to see in this case. You do not need to use a filter mapping rule in your telescope setup.

As Lew mentioned CBA imagers often take their images in a clear filter to reach fainter magnitudes. I recently ran DQ Her through VPhot in this fashion. First, set up your own imaging software to call the clear filter name either Clear or C. Either one works as you can see in the filter pull down box in the Image List. I happen to use C but I did use Clear at one time.

After you have uploaded your images, set up an appropriate sequence using AAVSO comps if possible. If you are doing a time series as I did, in the time series results page you will notice that the sequence pull down box will have a C. Open the box and change to V and click refresh. You will see the magnitudes change and be reported as V magnitudes. Look at your check star to confirm things are working reasonably well. The check value may differ somewhat from its known V mag because your image was taken in a C filter. 

Subsequently, click create AAVSO report (or general export). You will notice that your report in AAVSO format will show the filter as CV. If not, you did something wrong. Submit the report as normal.

Respond if you have any difficulties doing this.

Ken

Lew Cook
Lew Cook's picture
Clear vs C vs CV vs Luminance

In VPHOT, you can RENAME the filter to C. Click the images you want to change, click RENAME and RENAME all of those filter names  to "C". You can change the name of the telescope if it isn't one of yours in the list.

Lew

 

MZK
MZK's picture
Rename

Lew:

Thanks for mentioning that handy function. It is especially useful for iTelescope images that may be taken with a Luminance filter. It saves the need to change your image filter fits headers on your own computer. Ugh! VPhot does it in one quick step.

Ken

spp
spp's picture
CV magnitudes with clear filter

 

Thanks Lew and Ken.  I may be able to give this a try tonight, at least as a test. 

Phil

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